There exist two kinds of wireless network architectures, centralized and distributed. A conventional infrastructure-based cellular network can be considered a centralized network whereas ad hoc or mesh networks exemplify distributed networks. In a cellular network, a terminal such as, for example, a user equipment, UE, may communicate via a base station, BS, to obtain communication with another terminal. In an ad-hoc network, a terminal may communicate with another terminal bypassing base stations using direct traffic, or a relay. Such connections between terminals may be called device-to-device connections or direct device-to-device connections.
In a cellular network, traffic may traverse a centralized controller such as a base station or a switch even if the source and destination terminals are close to each other, for example in the same cell. In this case a terminal may transmit information to a base station, which forwards the information to a switch, which may return the information to the same base station for downlink transmission to the target terminal located near the terminal originating the information.
In an ad hoc network, an originating terminal form a direct radio connection to a target terminal and transmit information over a direct device-to-device connection, so that the information doesn't traverse any intermediate nodes along the way from the originating terminal to the target terminal. In cases where communicating terminals are geographically close to each other, they may accomplish communicating a given amount of information using less energy than would be required to communicate it via a cellular system. This is even more so when the terminals are simultaneously close to each other but far from a base station.
Combining ad hoc and cellular communications may provide a solution where a cellular network participates in coordinating the use of radio resources by ad hoc terminals. Radio resources may comprise, for example, frequency bands, time slots, spreading codes and other similar resources or their combinations. In such cases terminals are capable of communicating both with base stations and directly with other terminals.